I usually view it as a cost–benefit sort of thing. When a choice has nearly zero cost, only very little benefit is needed to make it the rational choice as far as I'm concerned.
What does WeeChat offer over irssi? Very active development, scripting in many languages, dynamic filtering, raw buffer views, sane defaults, helpful configuration interface, good documentation, live/hotswapping upgrades, architectural soundness, many options for interaction (FIFO pipes, client-server protocol) and so on and so forth. I'm always surprised at how little WeeChat gets in my way when I want to do something out of the ordinary.
And unless you are heavily invested in the irssi ecosystem, the switch is basically effortless.
That's the cost–benefit of it. Another way to view it is this: People who switch from Irssi to WeeChat keep using WeeChat; while people who switch from WeeChat to Irssi go back to WeeChat.
On a scale from ed to Office 365, WeeChat and irssi have practically the same default UI -- and it's butt-ugly. But I do see your point, and it's likely configurable. Anything specific you need help with?
